Question

Explain why it is difficult to make an X-ray laser.

Answer 1

Lasers typically require a mirror cavity to create a positive feedback for generating a large number of coherent (identical) photons. I case of X-rays, mirrors are very hard to fabricate and they only work for very small angles of incidence, and their reflectivity is well below 100%.

Another problem is high rate of spontaneous emission mentioned by another viewer. Basically, this causes the excitation of the laser medium to fizzle out before it can be extracted as a laser beam.

Finally, you need to somehow excite the laser medium. You could do this with higher energy X-rays but common sources of X-rays are somewhat similar to light bulbs and do not have sufficient brightness to overcome the issue of fizzling due to spontaneous emission. So far this could only be done in a pulsed mode - by vaporizing the material with a high-power laser beam, and obtaining X-ray emission from the subsequent recombination of ions and electrons.

hope this answers your question.